<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>第47个CSS页面</title>
</head>
<body>
<h4>外边距折叠（合并）</h4>

<div style="border: 1px solid red; width: 100px;">
    <div style="margin: 40px 0; background-color: green; height: 50px; width: 50px;">
        <div style="margin: 100px 0;">B</div>
    </div>
</div>

</body>
</html>